1. Field of the Invention
The present invention relates to a class-A amplifier and, more particularly, to a class-A amplifier circuit having output voltage capable of being rapidly and precisely varied as an input voltage changes.
2. Description of Related Art
Output voltages of conventional CMOS (complementary metal oxide semiconductor) class-A and class-AB amplifiers are shown in FIGS. 1 and 2. In FIG. 1, input voltage is boosted from a low level (e.g., VSS+2V≈2V) to a high level (e.g., VDD≈10V). On the contrary, in FIG. 1, input voltage is lowered from a high level (e.g., VDD≈10V) to a low level (e.g., VSS+2V≈2V). It is seen that there is no overshooting in the output voltage of the class-A amplifier in either figure. However, a relatively long period of time is required for either rising the output voltage of the class-A amplifier from the low level to the high level or lowering the same from the high level to the low level. This can limit an operating frequency of the class-A amplifier. Advantageously, only a relatively short period of time is required for either rising the output voltage of the class-AB amplifier from the low level to the high level or lowering the same from the high level to the low level. Unfortunately, the output voltage of the class-AB amplifier tends to overshoot, resulting in a generation of noise in the device.
Therefore, it is desirable to provide a novel amplifier circuit in order to mitigate and/or obviate the aforementioned problems.